As a general rule, an aircraft comprises a fuselage having a front portion in which a cockpit is arranged, a main portion that bears the wings and a rear fuselage that supports the horizontal and the vertical stabilizers.
Normally, the rear fuselage of the aircraft ends in a modular tail cone end. The rear fuselage houses the auxiliary power unit (APU) of the aircraft.
Thus, when assembling an aircraft, at the rear fuselage, the interface between the tail cone end and the rest of the rear fuselage is normally designed with four fittings or lugs, two upper ones and two lower ones, for supporting the loads, plus a balancer fitting capable of transmit loads and used also to align or centering the positioning of the different parts during the assembly process. Moreover, the balancer is used as a fail safe solution. This type of construction derives from the fail safe operability standards requested by the air navigation laws. Hence, in case of losing one of the fittings or lugs the aircraft always has four more in order to carry the loads.
The typical method for assembling the tail cone end to the rest of the rear fuselage follows these steps:                a) To install the balancer which guides and aligns the sections;        b) To install the lower fittings or lugs; and lastly        c) To install and adjust the upper fittings.        
The problem encountered by the above method is found in the final assembly line (FAL) in which the tail con end and the rear fuselage are joined. The problem is that once the first three fittings are adjusted (two lower ones plus the balancer according to points a) and b) above) problems to adjust the upper fittings are found due to tolerances and due to the elimination of the freedom degrees.
Traditionally the above problem was solved by adding a new step, before step c). Once the three lower fittings (two lower lugs plus balancer) are adjusted, the balancer is disassembled, then the upper fittings are adjusted and the balancer is re-assembled again. This solution involves expensive costs and an increase of time for the FAL as it is not part of the standard procedure for assembling the tail cone end to the rest of the rear fuselage.
In addition, since the rear fuselage houses the auxiliary power unit (APU) of the aircraft, the joint between the tail cone and the rest of the rear fuselage depends on the location of the air intake of the auxiliary power unit. Thus, in cases where the auxiliary power unit air intake is in a lower location, the balancer is in an upper position whereas in cases where the auxiliary power unit air intake is in an upper location, the balancer is in a lower position.